1. Field of the Invention
The present invention relates to balanced positioning systems. More particularly, the invention relates to such systems in lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
2. Discussion of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a “lens”. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatus are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions which are successively irradiated via the mask, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—which is commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In a lithographic apparatus, reactions on the machine frame to acceleration forces used to position the mask (reticle) and substrate (wafer) to nanometer accuracies are a major cause of vibration, impairing the accuracy of the apparatus. To minimise the effects of vibrations, it is possible to provide an isolated metrology frame on which all position sensing devices are mounted, and to channel all reaction forces to a so-called force or reaction frame that is separated from the remainder of the apparatus.
U.S. Pat. No. 5,208,497 describes a system in which the reaction of the driving force is channeled to a balance mass which is normally heavier than the driven mass and which is free to move relative to the remainder of the apparatus. The reaction force is spent in accelerating the balance mass and does not significantly affect the remainder of the apparatus. However, the concept disclosed in U.S. Pat. No. 5,208,497 is only effective for reaction forces in one direction and is not readily extendable to systems having multiple degrees of freedom. Balance masses moveable in three degrees of freedom in a plane are described in WO 98/40791 and WO 98/28665 (mentioned above).
EP-A-0,557,100 describes a system which relies on actively driving two masses in opposite directions so that the reaction forces are equal and opposite and so cancel out. The system described operates in two dimensions but the active positioning of the balance mass necessitates a second positioning system of equal quality and capability to that driving the primary object.
None of the above systems is particularly effective at counteracting yawing moments which may be induced by adjustments of the rotational position of the driven mass or because of misalignment between the line of action of forces exerted on the driven body and its center of mass.
U.S. Pat. No. 5,815,246 discloses a positioning system having a first balance mass free to move in an XY plane, i.e. to translate in X and Y and rotate about axes parallel to the Z direction. To control rotation of the first balance mass, a fly wheel, forming a second balance mass, is driven by a rotation motor mounted on the first balance mass to exert a counter-acting torque. Controlling rotation of the first balance mass therefore requires accurate control of the rotation and the flywheel. Any delay in this control or imbalance of the flywheel will cause vibration.